1. Field of the Invention
The invention relates to controls for industrial compressed air systems, especially systems in which a reciprocable motor is shifted between two end positions by a four-way control valve or a variant thereof. Conventionally, such systems have a three-way supply valve in the air line feeding the control valve, the supply valve being shiftable from time to time to its exhaust position in order to evacuate the system, and later shifted back to its supply position. This could result in sudden and dangerous shifting of the controlled part. For example, this part could be a press handling device which could drift by gravity to one position when line air is depleted and suddenly shifted back when full line pressure is applied.
2. Description of the Prior Art
It is known to place a piston-operated poppet safety valve between the supply and control valves, this safety valve being spring-urged to its closed position but having a restricted bypass from the supply valve to both the piston chamber and outlet ports of the safety valve. With this arrangement, full air pressure will be initially prevented from flowing from the supply to the control valve when the former is opened but will slowly build up in the safety valve piston chamber and simultaneously on one side of the reciprocable motor, slowly and safely shifting the motor to its opposite position. When the piston chamber pressure reaches a predetermined value, the safety valve will open, providing full supply pressure to the control valve.
In a known version of this safety valve, the restriction is in the form of a narrow hole drilled in the poppet valve member itself, with a restricted housing passage leading from the outlet port to the piston chamber.
This prior construction has disadvantages which it is an object of the present invention to overcome. It is necessary with the prior version to drill a separate hole in each poppet valve, and it has been found quite difficult to obtain satisfactory results in obtaining the right size of restriction, since extreme accuracy is required. Furthermore, it is impossible to vary the restriction size once the hole is drilled through the poppet valve member, such adjustability being often desirable.